re: "Is this a violation of the TOS? Are clients allowed to post jobs for fixed rate projects on Upwork to lure people to their personal agency off the platform?"
An Upwork client IS allowed to post a job on Upwork and then have you meet or organize with their team using non-Upwork methods, such as Slack or Facebook.
But the client MUST pay you through Upwork only.
A client may not post a job on Upwork and then ask you to work for free, or pay you through some other means outside of Upwork.
It is not entirely clear to me that the client you describe has violated Upwork ToS or not. What I would personally do is this:
If I was interested in doing the work she described, I would tell the client YES. I would say that I will do the work and ask her to send me an official Upwork Hire offer for either a fixed-price or hourly contract, whichever is appropriate for the work.
And then it is up to the client. If she hires you through Upwork, then that's great. You can do the work. If not, then you don't continue or do anything. Wait until you're hired.
I don't participate in team meetings with clients or their work groups PRIOR to actually being hired via Upwork.
And (me, personally) I don't get involved in projects on a FIXED-PRICE basis when there are meetings and communications involved. For me, that requires an hourly contract, through which I can bill for the time I spend in group meetings, team conferences, phone calls with clients, etc.
